Unfortunately theme components can’t change email styles, and we no longer support the box style.
Though one thing I now realize, is that we don’t use the text color category setting for the default category badges in Discourse anymore (it’s only used in theme components like the one you linked), though when updating the email I included some CSS that will use it… which is why your categories have white text in email. In order to be more consistent with the default category badges I should remove that.